The invention relates to methods and apparatus for electrically charging a substrate prior to coating a liquid composition to the substrate surface to form a coated layer thereupon; and more particularly, to methods and apparatus for controllably charging a moving web substrate prior to starting the application of an aqueous composition to the substrate surface; and most particularly, to methods and apparatus for providing a controlled high level of electrostatic charge to a web substrate for a brief time during initiation of application of a photographic composition and/or at splices between consecutive rolls of web substrate during a multiple-roll coating.
In coating a liquid composition to a moving web substrate, a critically important event is the first contact of the composition being delivered to the substrate surface at the start of coating, and the subsequent immediate formation of a stable coating relationship between the composition and the moving substrate surface. This event is especially complicated for multiple-layer bead coating from a multiple-slot slide hopper, as is well known in the art. Prior to commencement of coating, the multiple-layer composition is flowing down the hopper slide surface and over the hopper lip to a drain. Thus, the coating pack as presented to the web surface is inverted when the hopper is moved into coating position, the composition topcoat or overcoat being the first liquid to strike the web. The overcoat typically is not optimally coatable as the contact layer and may not readily wet the web surface. Further, the entire coating pack must re-invert virtually instantaneously into normal layer sequence as the composition begins to follow the web surface away from the lip. Further, the thick composition at the lip which typically is flowing downwards under gravity at a rate of a few feet per minute is instantaneously stretched and accelerated to web speed, which may be 1000 or more feet per minute. Further, the aerodynamic relationship of the hopper to the web changes as the bead is formed, the former coating gap is sealed, and a vacuum is formed under the lip. The superposition of these sub-events can result in a starting area of coating on the web which may be uneven, which may be many times thicker than at coating equilibrium, and which may feature stable widthwise thickness non-uniformities manifested as running streaks in the coating. Such streaks can render the entire coating defective. The thicker areas may not fully dry in the coating machine dryer before encountering face-side rollers to which the tacky composition may undesirably adhere and track off. At best, such a confused area of coating is discarded at finishing and thus represents coated waste.
Various approaches are known in the art to minimize the length of coating which must be discarded, either by minimizing thick coating starts, or by removing thick areas of coated composition from the web.
U.S. Pat. No. 4,340,621 to Matsumiya et al. discloses the use of a thin coating of a pretreatment liquid to the area of the web on which the coating start is to be made to improve wetting by the overcoat, combined with an increase in the under-lip vacuum level at the moment of composition contact with the web.
U.S. Pat. No. 5,154,951 to Finnicum et al. discloses a method and apparatus for immediate response and precise control of under-lip vacuum level to instantaneously and temporarily increase the level of under-lip vacuum.
U.S. Pat. No. 5,358,737 to Mues et al. discloses a method and apparatus for removal of non-dried thick areas of coating by brushing off the still-wet composition with a rotary brush prior to winding of the coated web.
U.S. Pat. No. 5,683,750 to Hoff et al. discloses the use of a low-viscosity temporary extra topcoat to increase wettability of the upper surface of the composition pack.
U.S. Pat. No. 5,700,524 to Hoff et al. teaches a shear-thinning topcoat to improve initial coatability.
U.S. Pat. No. 5,755,881 to Fenoglio et al. discloses method and apparatus for vacuum removal of excess composition from the web after a coating start while the composition is still liquid.
Another event of critical importance in multiple-roll coatings is coating across splices between rolls. Rolls are typically butt-spliced by a length of plastic adhesive tape which itself forms a ridge transverse to the coating bead. In addition, the joint between the webs can tend to fold as a hinge as it passes around the coating backing roller. Both of these effects can cause substantial disturbance of the coating bead, known in the art as xe2x80x9csplice blow-up.xe2x80x9d Studies have shown that a high percentage of running streaks result from bubbles generated by the splice disturbance and then dynamically trapped in the coating composition at the hopper lip.
An approach known in the art for reducing the severity of heavy starts and splice blow-up is to electrify the surface of the web to a high electrostatic voltage level in the start or splice area of the web onto which the composition is to be coated, just ahead of the coating station. See, for example, the relevant disclosures of U.S. Pat. No. 5,340,616 to Amano et al. and European Pat. No. 0 300 098 A1. Typically, an electrical charging apparatus having an array of exposed electrodes is disposed transverse to the direction of travel of the web. The charging apparatus, described more filly below, may or may not be in contact with the web surface. Charging of the web makes the web surface apparently more highly wettable and can provide highly even, relatively undisturbed coating starts and splices. A drawback of charging webs for photographic coating is that charging can fog the emulsion; therefore, electrification may be conducted solely to facilitate the start of coating or the coating of splices, and web areas so treated must be removed as waste at finishing.
Since the electrostatically treated and coated area must be discarded, it is desirable to minimize the length of treated web, typically to only a few feet. At high coating speeds, therefore, the charging apparatus must be energized for only a fraction of a second. However, the power supply for the apparatus cannot be set at a fixed power level which is optimal for all types of webs to be coated by a given coating apparatus. Webs differing in polymer composition, thickness, surface pretreatment, subbing layers, and coating speed can require substantially different power levels to achieve optimal charging. In principle, a feedback control system might be employed wherein a non-contacting voltmeter downstream of the discharge apparatus monitors the web face side charge and provides a feedback signal to a power supply driving the apparatus. However, the distance between the apparatus and the voltmeter represents a time lag in the system and thus an irreducible minimum length of uncontrollably electrified web. Clearly, a feedback system is not feasible when the length of web to be electrified is of the same order as the distance between the power supply and the voltmeter.
In actual prior art practice, the settings for the power supply are made manually by an operator and are only experiential starting points which generally require modification during the course of a coating run. Settings may be varied manually by an operator based on visual inspection of prior samples in a run, rather than on an understanding and implementation of the fundamental relationship of the charging apparatus, the web to be charged, the web voltage level required, and the coating speed, which will lead to optimal charging of each web type.
The prior art fails to teach a system for automatically setting and controlling the output of the power supply of a charging apparatus to the optimal level for the proper length of time for any web being coated at any coating speed, based upon predetermined web, apparatus, and coating parameters.
It is therefore an object of the present invention to provide an improved programmably controlled web charging system for automatically providing an optimal level of electrostatic charge to web substrates at the start of coating and at splices to minimize heavy coating starts and splice blow-up.
It is a further object of the present invention to provide an improved web charging system including a programmable controller for controlling the output of a power generator.
It is a still further object of the present invention to provide an improved web charging system wherein a programmable controller utilizes an algorithm which models the charging system and is responsive to operational parametric input.
Yet another object of the present invention is to provide an improved programmably controlled web charging system for automatically providing an optimal level of electrostatic charge to web substrates which can be used in conjunction with a variety of different coating processes including, but not limited to, bead coating, curtain coating and extrusion hopper coating.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by using a charging apparatus for depositing controlled bursts of electrostatic charge on a moving web to be coated. Control of the deposition of electrostatic charge is performed using a mathematical model that estimates the charging performance of the charging apparatus. The model is constructed via benchtop characterization of the apparatus and of the webs to be coated. The model is implemented in coating production via either an algorithm comprising a best-fit equation or a lookup table, representing the model predictions over a range of relevant input parameter values such as web speed, web capacitance, and desired web voltage.
The apparatus includes a web charger, a power supply for powering the charger, and a controller programmed with the algorithm for automatically setting and controlling the intensity and duration of the output of the power supply. The web charger includes electrode arrangements, such as an array of electrodes, capable of creating sufficiently high electric fields when raised to high voltage so as to exceed the breakdown limit of air. This results in creation of positive and negative electrical charges that are then available for deposition onto the moving web substrate. Such arrangements may include wires, pins, brushes, and blades. Typically, these are non-contacting of the web to prevent mechanically damaging the web surface. Arrangements may also include a charged roller or rollers in contact with the web and which can create high electric fields at the entering or leaving nip.
In operation, run-specific variables including web type, coating speed, desired voltage level on the web, activation signal, and length of charging period are provided as inputs to the controller.
The term xe2x80x9cweb substratexe2x80x9d or xe2x80x9cwebxe2x80x9d as used herein comprises a flexible, planar sheet formed of plastic polymer, paper, or metal. Such web substrates are typically the supporting element in photographic films, papers, and photoengraving products. Webs are generally of a predetermined width and may be variable or indefinite in length. They may be non-coated when subjected to the treatments described herein, or they may include one or more previously coated layers.